Mary E. Power

Trophic Downgrading of Planet Earth

Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

Challenges in the quest for keystones

Mary E. Power is a professor in the Department of Integrative Biology, University of California, Berkeley, CA 94720. David Tilman is a professor in the Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108. James A. Estes is a wildlife biologist in the National Biological Service, Institute of Marine Science, University of California, Santa Cruz, CA 95064. Bruce A. Menge is a professor in the Department of Zoology, Oregon State University, Corvallis, OR 97331. William J. Bond is a professor doctor in the Department of Botany, University of Cape Town, Rondebosch 7700 South Africa. L. Scott Mills is an assistant professor in the Wildlife Biology Program, School of Forestry, University of Montana, Missoula, MT 59812. Gretchen Daily is Bing Interdisciplinary Research Scientist, Department of Biological Science, Stanford University, Stanford, CA 94305. Juan Carlos Castilla is a full professor and marine biology head in Facultad de Ciencias Biologicas, Pontificia Universidad Catolica de Chile, Casilla 114-D, Santiago, Chile. Jane Lubchenco is a distinguished professor in the Department of Zoology, Oregon State University, Corvallis, OR 97331. Robert T. Paine is a professor in the Department of Zoology, NJ-15, University of Washington, Seattle, WA 98195. ? 1996 American Institute of Biological Sciences. A keystone species is

Effects of Fish in River Food Webs

Experimental manipulations of fish in a Northern California river during summer base flow reveal that they have large effects on predators, herbivores, and plants in river food webs. California roach and juvenile steelhead consume predatory insects and fish fry, which feed on algivorous chironomid larvae. In the presence of fish, filamentous green algae are reduced to low, prostrate webs, infested with chironomids. When the absence of large fish releases smaller predators that suppress chironomids, algal biomass is higher, and tall upright algal turfs become covered with diatoms and cyanobacteria. These manipulations provide evidence that the Hairston, Smith, Slobodkin–Fretwell theory of trophic control, which predicts that plants will be alternately limited by resources or herbivores in food webs with odd and even numbers of trophic levels, has application to river communitics.

Grazing minnows, piscivorous bass, and stream algae: dynamics of a strong interaction

Striking differences in pool-to-pool distributions of an algae-grazing minnow (Campo- stoma anomalum), attached algae (predominantly Spirogyra sp. and Rhizoclonium sp.), and bass (Micropterus salmoides and M. punctulatus) are known to occur in some small Oklahoma streams. This study evaluates the complementarity of bass, Campostoma, and algae at different seasons, and uses in-stream experimental manipulations of bass and Campostoma to determine if the patterns resulted from strong interactions between predators, herbivores, and algae. In a 1-km reach of Brier Creek (south-central Oklahoma), bass and Campostoma distributions in 14 consecutive pools were inversely related in six of seven censuses conducted from 8 November 1982 to 5 September 1983. Bass and Campostoma co-occurred in more than two pools only on two occasions, following the largest floods of the year. Campostoma and algal abundances were inversely related during late summer and in both autumns of this study. This relationship did not hold during the spring, when floods strongly affected algal distributions. During autumn of 1983, we removed bass from a pool, fenced it longitudinally, and added Campo- stoma to one side (1.4 individuals/m2). Over the next 5 wk, standing crop of algae decreased significantly on the Campostoma side but increased on the control side. In a nearby unmanipulated Campostoma pool, standing crop of algae was consistently low. We added three free-swimming bass to a Campostoma pool to evaluate presumptive predator-prey interactions. Within 3 h, the Campostoma moved from the deepest part of the pool to shallow areas. Over the next 5 wk, numbers of grazing Campostoma declined due to behavioral changes, emigration, and (presumably) predation. The standing crop of algae increased significantly 10-13 d after bass addition. In a second bass-addition experiment in June 1984, Campostoma responses were almost identical, and algal standing crop in deeper areas increased significantly after 1 wk. Collectively, our censuses and the experiments indicate that in Brier Creek, biotic interactions strongly influence the pool-to-pool distributions of Campostoma and algae, par- ticularly during long periods of constant low discharge.

Filling key gaps in population and community ecology

We propose research to fill key gaps in the areas of population and community ecology, based on a National Science Foundation workshop identifying funding priorities for the next 5-10 years. Our vision for the near future of ecology focuses on three core areas: predicting the strength and context-dependence of species interactions across multiple scales; identifying the importance of feedbacks from individual interactions to ecosystem dynam- ics; and linking pattern with process to understand species coexistence. We outline a combination of theory devel- opment and explicit, realistic tests of hypotheses needed to advance population and community ecology.

Mountaintop Mining Consequences

Damage to ecosystems and threats to human health and the lack of effective mitigation require new approaches to mining regulation. There has been a global, 30-year increase in surface mining (1), which is now the dominant driver of land-use change in the central Appalachian ecoregion of the United States (2). One major form of such mining, mountaintop mining with valley fills (MTM/VF) (3), is widespread throughout eastern Kentucky, West Virginia (WV), and southwestern Virginia. Upper elevation forests are cleared and stripped of topsoil, and explosives are used to break up rocks to access buried coal (fig. S1). Excess rock (mine “spoil”) is pushed into adjacent valleys, where it buries existing streams.

Structural and Functional Loss in Restored Wetland Ecosystems

In restored wetland ecosystems with apparently natural hydrology and biological structure, biogeochemical function may remain degraded, even a century after restoration efforts.

Biotic and Abiotic Controls in River and Stream Communities

Lotic ecologists share a major goal of explaining the distribution and abundance of biota in the worlds rivers and streams, and of predicting how this biota will respond to change in fluvial ecosystems. We discuss five areas of research that would contribute to our pursuit of this goal. For mechanistic understanding of lotic community dynamics, we need more information on: 1. Physical conditions impinging on lotic biota, measured on temporal and spatial scales relevant to the organisms. 2. Responses of lotic biota to discharge fluctuations, including the processes that mediate community recovery following resets caused by spates or droughts. 3. Movements of lotic organisms that mediate gene flow, resource tracking, and multilevel species interactions. 4. Life history patterns, with special emphasis on ontogenetic bottlenecks that determine the vulnerability of populations confronting environmental perturbation. 5. Consequences of species interactions for community- and ecosystem-level processes in rivers and streams. Without attempting to be comprehensive in our review, we discuss limits and limitations of our knowledge in these areas. We also suggest types of data and technological development that would advance our understanding. While we appreciate the value and need for empirical and comparative information, we advocate search for key mechanisms underlying community interactions as the crucial step toward developing general predictions of responses to environmental change. These mechanisms are likely to be complex, and elucidation of interacting bilateral, or multilateral, biotic and abiotic controls will progress only with the continuing synthesis of community- and ecosystem-level approaches in lotic ecology.

Effects of Disturbance on River Food Webs

A multitrophic model integrating the effects of flooding disturbance and food web interactions in rivers predicted that removing floods would cause increases of predator-resistant grazing insects, which would divert energy away from the food chain leading to predatory fish. Experimental manipulations of predator-resistant grazers and top predators, and large-scale comparisons of regulated and unregulated rivers, verified the model predictions. Thus, multitrophic models can successfully synthesize a variety of ecological processes, and conservation programs may benefit by taking a food web perspective instead of concentrating on a single species.

DEFORMATIONAL MASS TRANSPORT AND INVASIVE PROCESSES IN SOIL EVOLUTION

Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above.